This invention is addressed to a process for the extraction of nonpolar constituents from natural substances such as hops, with the simultaneous separation of the residues of nonpolar plant-protecting agents. The hop plant is named in this connection only by way of example, since the problem of removing undesirable residues occurs in numerous natural substances, whether the starting substances are cultivated or wild.
The constituents which determine the value of hops are the hop resins and hop oils. The most important hop resins, the .alpha.- and .beta.-picric acids, as well as the other soft resins, are of a nonpolar, lipophilic character, and so are the terpenes and sesquiterpenes of hop oil. Since all these substances are relatively unstable, there are many possibilities for separating and concentrating the important constituents of vegetable matter by extraction with suitable solvents (on the state of the art, see for example SANDER, W. and DEUBLEIN, B., in Brau-Industrie 1982, p. 997).
Such extractants are organic solvents, especially dichloromethane, hexane, methanol, or ethanol. Recently, the extraction of hops with highly compressed CO.sub.2 has been described. In the process according to German Federal Patent No. 21 27 618, CO.sub.2 at supercritical pressure and temperature is used as the extractant. The separation of the desired substances from the solvent is then performed by reducing the density by lowering the pressure with simultaneous evaporation of the CO.sub.2. The CO.sub.2 extracts thus obtained are regarded in the industrial field as especially pure and stable.
Hops are treated during their growing season from March to August with numerous plant protectives which ultimately always lead, regardless of their form, to residues, even though sometimes in minute amounts. In the Federal Republic, about 40 plant protectives are presently approved for hop growing.
The residues of these plant protectives are understandably undesirable in every case, as is underlined by their intensive public discussion. Raw materials for beer-making are especially subject to critical evaluation, since beer has proven, on account of the demand for purity, to be a very sensitive food substance. Consequently, it would be considered advantageous if it were possible to produce hops and hop products of very low residue content.
It is already possible to reduce the amount of residues that occur by developing and growing disease-resistant types of hops, but freedom from residues is an unattained goal. A partial reduction can also be achieved by solvent extraction. The more selective a solvent is, the fewer are the residues that can be extracted. Results have been published on dithiocarbamate residues, for example, in Brauwelt, 1981, 825 (Nitz, S. et al.), and on heavy metals in Brauerei-Rundschau, Vol. 92, July 1981, No. 7 (Schur, F. et al.). Even though it is to be assumed that the very selective and nonpolar solvent carbon dioxide has the advantage over ethanol, for example, that it does not dissolve plant protectives of a polar nature, there are a number of nonpolar protectives which can be dissolved out of the plants by CO.sub.2 extraction. For example, in Planta Medica No. 2, April 1984, pp. 171-173, there is a report on the possibility of removing plant protectives from drugs with supercritical carbon dioxide. DDT and hexachlororcyclohexane as lipophilic constituents are extracted at relatively low pressures (80-120 bar) from drugs whose value-determining substances are extracted only at higher pressures (above 150 bar), or which have, so to speak, a polar character whereby they are indissoluble in CO.sub.2. The authors come to the conclusion that the process described is applicable only to plants which do not contain lipophilic substances such as ethereal oils, for example.
However, the substances obtained from hops are lipophilic, and thus the process described in Planta Medica is not applicable to hops.
Attempts to reduce residue content have been performed with three important representatives of plant protectives:
1. Folpet (N-(trichloromethylthio)-phthalimide) and
2. Metalaxyl (D,L-N-2,6-dimethylphenyl-N-(2'-methoxyacetyl)alanine methyl ester)
as fungicides, and
3. Endosulfan (6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzod ioxathiepin 3-oxide)
as an insecticide.
Studies with folpet insecticide (N-(trichloromethylthio)phthalimide), which is used preferentially against fungus infections by Peronospora, Phomapsis, Fusicladium, Botrytis, etc., have shown that under all conditions known heretofore, both in the liquid range (e.g., 70 bar, 15.degree. C. or 150-300 bar, 25.degree. C.) and in the supercritical range (150-300 bar, 40-80.degree. C.), extraction with CO.sub.2 is successful. Upon the necessary separation, therefore, the extract and the folpet occur together. Decontamination can be performed at 80-100 bar and 60.degree. C. from, for example, 50 ppm in the starting hops, to 0.5 ppm. However, the advantage of the 99% removal of the residue is offset by the disadvantage that about 10 to 20% of the soluble extract goes with it. The separation of this extract containing the folpet leads to a product in which the residue is greatly concentrated, and which must be discarded. The extraction of the active substances from the hops thus decontaminated can then be performed under normal extraction conditions. Therefore, while on the one hand a virtually residue-free extract is produced, on the other hand there is an unacceptable economic disadvantage in the form of the extract produced by the decontamination.
In sum, the use of plant-protective agents (insecticides, fungicides, herbicides) is unavoidable, at least with monocultures and under unfavorable climatic conditions. It may thus be desirable, or even necessary, to separate these poisonous substances from the materials of value used in the food area. This is easily possible, provided that the materials of value are not dissolved at the same time as the poisonous substances during an extraction. However, if both groups of materials show a similar behavior toward the solvent, a separation in this simple manner is not possible.